Joi: "tel" is not allowed to be empty

Joi is returning the following error even though tel is set to be optional. How do we fix this?

Thanks.

Error: Joi Failed: ValidationError: child "tel" fails because ["tel" is not allowed to be empty]

//Define Joi schema
const schema = {
    email: Joi.string().required().email({
        errorLevel: 64,
        minDomainAtoms: 2 
    }).min(6),
    tel: Joi.string().optional().min(10).max(10),
    password: Joi.string().required().min(8).max(64)
}

//Check inputs
const { error, value } = Joi.validate({ 
    email: args.email, 
    tel: tel, 
    password: args.password 
}, schema)